1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In recent years, digital printers employing electrophotographic process have been popularly used in the field of production printing. Higher resolution and higher reliability are thus required of digital printers employing the electrophotographic process. In particular, for digital printers employing the electrophotographic process, the improvement in the accuracy of positioning between a front side and a back side (registration) is required. In the digital printer employing the electrophotographic process, shrinking of sheets during thermal fixing process occurs. So that if when no process is applied to the image data in two-sided printing, misregistration between the front side and the back side occurs.
To solve the problem, the digital printer performs position correction of input image data such as rotation, scaling, or shifting. For an accuracy of the process, most of digital printers perform position correction of multi-bit data with high resolution of, for example, 1200 dpi or 2400 dpi.
The digital printer employing the electrophotographic process includes a photoconductor drum having a photoconductive scan surface, a light source which emits a laser beam, a polygon mirror for deflecting the laser beam emitted from the light source, and a scanning optical system for introducing the laser beam deflected by the polygon mirror to the surface of the photoconductor drum (scan surface). The digital printer employing the electrophotographic process modulates the light flux emitted from the light source based on image data and irradiate the scan surface with the resulting light flux so as to scan the scan surface to produce an electrostatic latent image corresponding to the image data on the photoconductor drum.
An element having a plurality of luminous points, such as a laser diode array (LDA) and a vertical cavity surface emitting laser (VCSEL), is used for the light source of the digital printer employing the electrophotographic process. With this configuration, the digital printer employing the electrophotographic process can produce an electrostatic latent image with resolution, for example, 2400 dpi or 4800 dpi, which is higher than that of image data with 1200 dpi.
The digital printer performs processing of an image with such high resolution to precisely rotate, scale, or shift the image. The digital printer can thus perform precise registration between the front and back sides through the image processing with high resolution.
When image processing is performed with high resolution, however, a problem occurs when transferring data to the light source driving circuit in the downstream of the image processing unit. For example, when the image processing unit performs processing of multi-bit data with the resolution of 2400 dpi or 4800 dpi, flexibility of image processing is increased and reproducibility of small letters and lines of 1200 dpi is also increased. However, the high density image processing significantly increases the amount of data to be transferred to the light source driving circuit provided in the downstream of the image processing unit, which limits the rate of production and thus productivity.
JP 4622850 B discloses a technique to perform processing with high resolution in a required section through performing correction by two stages, that is, pre-correction and post-correction, so as to reduce the required memory. The technique disclosed in JP 4622850 B however cannot reduce the amount of transferred data.
JP 5017910 B and JP 4882426 B disclose techniques of performing correction after gradation processing. In the processing of registration between the front and back sides after the gradation processing, however, the problem of interference with the dithering disallows correction with a large dynamic range.
In view of the above, there is a need to provide an image forming apparatus that performs precise position correction of an image before gradation processing and performs image processing with high resolution without increasing the amount of transferred image data.